In steam turbines, steam is used as flow medium which can have a temperature of over 600° C. at a pressure of over 300 bar. Such high temperatures and pressures make increased demands upon the materials of the steam turbine. In particular, the region of the steam admission is thermally and mechanically highly stressed.
A steam turbine customarily has an inner casing, an outer casing which is arranged around the inner casing, and a rotor which is rotatably mounted inside the inner casing. The live steam flows through the outer casing and the inner casing into a flow passage to reach the rotor. The coaxial interface regions between the casings are thermally highly stressed and in relative displacement.
To accommodate differential thermal expansion of the casings while substantially preventing the leakage of high pressure steam therebetween it is common practice to provide a slip-type expansion joint of the type shown in FIG. 1.
The internal and external casings 1, 2 have wall openings, coaxial with corresponding fluid passages 3, 4, defining a cylindrical recess with opposing axial surfaces 102, 201 acting as seat for the joint. A sealing ring 5 is disposed in the recess such as to axially and radially engage with the recesses 102, 201 around the fluid passages 3, 4. A bushing 7 is tightened in the inner casing 1 to form an abutting conduit having a retaining ring 107 axially blocking the sealing ring 5.
Due to unsteady operation characterized by high pressure and high temperature gradient, causing differential thermal expansion and contraction and strong turbulent vibrations, the interfaces between the casings are in relative displacement when in use. To prevent rotation, the seat 201 on the inner casing 1 has a shape mating a corresponding section of the sealing ring 5. As shown in FIG. 2a. In operation, due to high temperature, high pressure, high velocity and turbulence of the steam, the sealing ring tends to vibrate and rotate. This determines a non perfectly uniform gap 401 between the seat and the ring as shown by the arrows in FIG. 2b causing a lateral wear of the seat in those zones 401 having reduced clearance (see FIG. 2c). This solution is thus non robust with respect to rotation.
An alternative to a squared seat joint is the device disclosed in U.S. Pat. No. 2,863,632. The slip ring expansion joint of this document is, however, rather complex and expensive as it requires a multiple ring pack configuration.
Therefore there is a need for an expansion joint for sealingly connecting two turbine components that is robust to anti-rotation behavior and at the same time simple, inexpensive and ease of installation also to retrofit existing/damaged units.